Now that millionaire space tourist Dennis Tito has officially announced his intention to send a man and a woman on a round-trip journey to Mars in 2018, a few more details are trickling out about the Inspiration Mars project. One such detail is how the Inspiration Mars team plans to protect its astronauts from cosmic radiation: by lining the spacecraft walls with water, food, and the astronauts' own feces.

Taber MacCallum, co-founder and CEO of the Paragon Space Development Corporation and member of the Inspiration Mars team, told the New Scientist that they plan to use a technology based on Water Walls Architecture, a project devised by NASA's Innovative Advanced Concepts program. The idea is to line the walls of a spacecraft with food and water, which will gradually be replaced by dehydrated waste. (The reclaimed water will be recycled and consumed by the astronauts.) Water, MacCallum explains, serves as a better radiation shield than metal does, because it's the nuclei of atoms that block the radiation, and water contains more atoms—and therefore more nuclei—per volume than metal does. Food and waste also provide good radiation shielding, and because the food blocks rather than absorbs the radiation, it will remain safe to eat.

This isn't a new idea. Back in 2011, Michael Flynn, a life support engineer at NASA Ames Research Center, discussed the possibility of using urine and feces to shield space stations, and Packing for Mars author Mary Roach also mentioned it in a 2011 edition of The Geek's Guide to the Galaxy. MacCallum notes, however, that they are still working out many aspects of the system, including figuring how best to keep the Mars-bound couple from experiencing too many nasty sights and smells on their journey.